The present invention relates to improved cutting inserts for grooving and parting off tools with a holder and a replaceable cutting insert.
Grooving and parting off tools serve as turning tools in lathes for effecting grooving and parting off operations. During these operations, the grooving and parting off tool is radially moved towards the workpiece, while the latter is not submitted to any axial movement. In this way a groove is formed in the workpiece, and eventually the workpiece is cut off, when the groove reaches the axis of rotation. Due to obvious reasons, the groove should have a small width for the parting off operation. Therefore, the cutting insert also is narrow. The same is also true for the part of the tool holder that is introduced into the groove during machining, i.e., the support blade. This blade is usually made even thinner than the cutting insert, in order to keep the blade away from the walls of the groove.
The cutting insert comprises a front cutting head and a rear clamping part for clamping the cutting insert in the support blade.
Several cutting inserts of this type are known. As examples can be mentioned SE-B-377 771, SE-B-454 060, SE-B-461 635, U.S. Pat. No. 4,969,779 and U.S. Pat. No. 5,135,336. All of these references belong to the same art as the present invention, i.e., to cutting inserts for grooving and parting off operations. Furthermore, they all comprise a recess or depression on the chip surface, so that the chips intrude into this recess and, seen in a cross-section of a chip, curve into the recess, thus decreasing the width of the chip and rendering the chip smaller than the width of the groove.
Although the known cutting inserts usually work satisfactorily, they are marred by a few inconveniences. Thus, the combined knowledge of the prior art supports the theory that the difference between the chip-breaking angle over the depression and the corresponding angle over the side surfaces of the depression is too slight. This small angle may cause too insignificant a deformation of the chips, which, in turn, can result in chip jamming and damaging of the groove surfaces. Moreover, the zero-degree or positive rake angles of the exposed side parts of the cutting edge, at both sides of the depression, occasionally cause untimely wearing out and splintering of these side parts.
Eventually, cutting inserts comprising a depression which intersects the cutting edge create an uneven bottom surface of the groove at grooving operations, which is undesireable or unacceptable for several applications. Naturally, this vaulted deformation of the groove bottom is exaggerated as the diameter of the groove bottom is smaller.
Hence, an object of the present invention is to construct a cutting insert which brings about a safe and sufficient decrease of the chip width and causes short chips, and which comprises strong side part cutting edges. A further object of the present invention is to provide a cutting insert that will produce an even and smooth groove bottom surface.
According to the present invention, these objects and others are realized by providing the side chip surfaces with a consistently negative rake angle, and providing the side of the chip-forming depression next to the cutting edge with a consistently positive rake angle.